1. Technical Field
The present invention relates to a transmission system operative to transmit packets via a network.
2. Description of the Related Art
TV broadcasting and other conventional video delivery services utilize leased circuits, such as ATM (Asynchronous Transfer Model) networks. Lately developed video delivery services, on the other hand, utilize virtual leased circuits, such as the Internet or wide area Ethernet (registered trademark). The Internet and the wide area Ethernet are based on the Ethernet technique and allow extremely higher-capacity networks to be established at relatively low cost, compared with the conventional ATM network.
The Ethernet, however, has the problems of poor communication quality, for example, ‘frequent packet losses’ and ‘significant delay fluctuation’, which less arise in the ATM network. Delivery via the Internet or another network based on the Ethernet technique may cause a TV broadcast video to have flicker or accidental interruption in the course of broadcasting.
The prior art packet transfer technique generates multiple copies of each packet as a transfer object and respectively sends the generated multiple copies over multiple transmission paths. In the event of some fault or trouble arising on one transmission path, the pathway for packet transmission is switched over to another transmission path with no occurrence of fault or trouble. In the case of a switchover of the transmission path in response to detection of a fault or a trouble by high-speed fault detection technique, one frame of packets may be missing in a video stream. An instantaneous interruption-free switchover of the pathway for packet transmission on the occurrence of some fault or trouble on one transmission path would thus be highly demanded to prevent such packet missing.
One proposed technique for attaining instantaneous interruption-free switchover of the transmission path is disclosed in Japanese Patent Laid-Open No. H09-135228. FIG. 17 conceptually illustrates the configuration of a prior art transmission system 3000P disclosed in this cited reference. The prior art transmission system 3000P includes a transmitter device 100P and a receiver device 200P. The transmitter device 100P has a replication module 102P, which assigns characteristic overhead information as a mark to each user packet input from a user terminal 1000 to capsulate the packet with the overhead information, replicates the capsulated packet to generate replicated packets, and outputs the replicated packets to a transmission path 404 or an A pathway and to a transmission path 405 or a B pathway. The receiver device 200P includes a phase adjuster 214P and a selector 206P. The phase adjuster 214P equalizes the phases of packets respectively received over the two transmission paths 404 and 405 and thereby synchronizes the received packets. The selector 206P selects a normal packet, for example, a packet with no missing or a packet with no falsification, out of the synchronized packets and outputs the selected packet as a user packet to another user terminal 2000.
FIG. 18 is a conceptual view showing a transmission path switchover process in the prior art transmission system of the above cited reference using leased line service, such as ATM networks, for the transmission paths. Each solid rightward arrow represents a time axis of the A pathway or the B pathway and indicates that the righter side represents the older time (the greater past time). The column (a) of FIG. 18 conceptually shows packet intervals in the A pathway and in the B pathway. As illustrated, a stream of sequential packets transferred over the A pathway is received by the receiver device 200P earlier than a corresponding stream of sequential packets transferred over the B pathway. As is known, the ATM network has the extremely high network quality and little delay fluctuation. The phase difference of the packets received by the phase adjuster 214P (FIG. 17) is expected to be only a fixed phase difference caused by a difference between the lengths of the respective transmission paths as shown in the column (a) of FIG. 18.
The column (b) of FIG. 18 shows phase adjustment of packets with time. As mentioned above, the phase difference of the packets received by the phase adjuster 214P is expected to be only the fixed phase difference caused by the difference between the lengths of the respective transmission paths. Insertion of a fixed delay to the earlier-received stream allows synchronization of adjusting the phase of the earlier-received stream to the phase of the later-received stream. The phases of the packets are readily adjustable in this manner as shown in the column (b) of FIG. 18.
The column (c) of FIG. 18 shows output of packets. The synchronization of the phases of the packets transferred over the A pathway and over the B pathway causes no packet missing in the event of a switchover of the transmission path. This readily attains instantaneous interruption-free switchover as shown in the column (c) of FIG. 18.
FIG. 19 is a conceptual view showing a transmission path switchover process in the prior art transmission system of the above cited reference using networks based on the Ethernet technique, such as the Internet or wide area Ethernet, for the transmission paths. The column (a) of FIG. 19 conceptually shows packet intervals in the A pathway and in the B pathway. The column (b) of FIG. 19 shows phase adjustment of packets with time. The column (c) of FIG. 19 shows output of packets.
In a network based on the Ethernet technique, a significant delay fluctuation may occur in the course of packet transmission. In the illustrated example, there is a delay fluctuation in the B pathway as shown in the column (a) of FIG. 19. In the phase adjuster 214P, the phase difference between a packet transferred over the B pathway and a corresponding packet transferred over the A pathway may vary among the respective packets.
In such a case, insertion of fixed delays to the earlier-received stream for synchronization of adjusting the phase of the earlier-received stream to the phase of the later-received stream causes the packets transferred over the A pathway at a fixed packet interval to have the same delay fluctuation as that occurring in the B pathway as shown in the column (b) of FIG. 19. An output stream of packets output from the transmission system may accordingly have a partially varying packet interval or a burst (intermittent transmission of a group of packets). In the illustrated example, packets 3 through 6 have a burst as shown in the column (c) of FIG. 19. The transmission system may thus worsen the state of packet transmission. In the condition of the varying phase difference among the respective packets as shown in the column (b) of FIG. 19, phase adjustment by insertion of fixed delays is technically difficult.